1. Field of the Invention
The invention pertains to fabrication of semiconductor devices, and in particular to fabrication techniques using an antireflective bi-layer hardmask for etching polysilicon.
2. Related Technology
An emphasis on increased semiconductor device performance has led to investigation of ways to increase device speed. One way of increasing device speed is to reduce the size of individual circuit components and the wiring that connects them. This enables circuit components to operate faster and to be placed closer together, and enables more circuit components to be used in a given device.
A recently developed technique uses an antireflective bi-layer hardmask structure as shown in FIG. 1 for patterning narrow polysilicon features. In FIG. 1, a substrate 10 has formed thereon a silicon oxide gate insulating layer 12 and a polysilicon gate conductive layer 14 that are to be patterned to form a gate line and gate insulator. A bi-layer hardmask structure including an amorphous carbon layer 16 and a protective layer 18 such as silicon nitride, silicon oxynitride or silicon oxide is formed over the polysilicon layer 14. A photoresist mask 20 is formed on the protective layer 18. The photoresist mask 20 is used to pattern a first hardmask from the protective layer 18, which in turn is used to pattern a second hardmask from the amorphous carbon layer 16, which in turn is used to pattern the polysilicon layer 14.
The bi-layer hardmask structure of FIG. 1 has a number of advantages. One advantage is that the bi-layer hardmask serves as an antireflective coating (ARC) when the amorphous carbon layer 16 is formed to a thickness of approximately 200-1000 angstroms and the protective layer is formed to a thickness of approximately 100-500 angstroms. This enables accurate patterning of overlying photoresist down to the minimum feature size achievable through the projection lithography technique being used. A second advantage of the bi-layer hardmask using amorphous carbon is that the photoresist mask and the first and second hardmasks may each be subjected to a trimming process whereby a portion of the mask is consumed by an isotropic etching plasma, thus narrowing the width of each mask. Such a sequential reduction of mask sizes enables the patterning of polysilicon features having feature sizes that are significantly smaller than the smallest feature size that can be created in the initial photoresist mask. A further advantage of the antireflective bi-layer hardmask is that the remaining hardmask materials can be selectively removed from the final patterned structure by ashing the amorphous carbon layer in an ashing atmosphere such as an isotropic oxygen or hydrogen plasma, which removes the amorphous carbon and detaches any remaining overlying material without damaging the underlying polygate structure and gate oxide.
Despite these advantages, other aspects of the antireflective bi-layer hardmask of FIG. 1 present obstacles to accurate pattern transfer. One obstacle is that the chlorine or HBr etch chemistry used to pattern polysilicon consumes amorphous carbon at a relatively high rate. The selectivity rate for amorphous carbon relative to polysilicon is approximately 1:3. Therefore, given the need to use an amorphous carbon thickness of approximately 500 angstroms in order to achieve antireflective properties, a maximum of approximately 1500 angstroms of polysilicon may be etched using a 500 angstrom amorphous carbon hardmask before so much of the amorphous carbon is consumed that pattern transfer accuracy is degraded. This problem is illustrated in FIGS. 2a-2b. FIG. 2a shows an amorphous carbon hardmask 22 having a thickness of approximately 500 angstroms that is formed on a polysilicon layer 14 having a thickness of approximately 2000 angstroms. As shown in FIG. 2b, by the time the polysilicon has been completely etched to form a polysilicon gate 24, the material of the amorphous carbon hardmask 22 is nearly depleted, causing its width to significantly narrow and thus producing a tapered profile in the polysilicon gate 24. In some instances, the loss of amorphous carbon may result in a gate that is too thin, causing a pattern collapse. If the amorphous carbon is consumed completely, then detrimental effects including top rounding, line edge roughness and a reduction in the height of the gate line will result. Since the amorphous carbon layer thickness may vary across the substrate topography, such inaccuracies are likely to occur at thinner portions of the amorphous carbon layer. Another obstacle presented by the antireflective bi-layer hardmask involves poor adherence of amorphous carbon to other materials and differences in the mechanical properties of amorphous carbon and polysilicon. In practice, the internal compressive forces generated within the amorphous carbon are contained so long as the overlying protective material is present. However, when the overlying protective layer is removed during patterning of a narrow line, the remaining adhesive forces between the polysilicon and amorphous carbon, which are relatively weak, may not be sufficient to contain the expansion forces of the amorphous carbon. In the instances where the length of the amorphous carbon line is much larger then its width, the amorphous carbon hardmask expands and delaminates from the polysilicon, causing lengthening where delamination occurs. Thus the expansion of the amorphous carbon degrades pattern transfer accuracy and must therefore be avoided
A third obstacle presented by the antireflective bi-layer hardmask structure of FIG. 1 is photoresist poisoning and premature amorphous carbon etching. Protective layer materials such as silicon nitride and silicon oxynitride generally contain “pin holes.” The pin holes are believed to be formed during protective layer deposition by outgassing from the amorphous carbon or incomplete coverage of nucleation sites. During photoresist application, the pin holes in the protective layer provide a path for “poisoning” of undeveloped photoresist by contaminants from the amorphous carbon layer such as nitrogen or other dopants that may be used to enhance etch selectivity. Contaminated photoresist may not react appropriately with developer chemistry, resulting in photoresist pattern abnormalities that will be transferred to underlying layers during later processing. Further, the pinholes enable photoresist developer and rework chemistry to attack the amorphous carbon, producing amorphous carbon pattern abnormalities.